Mars atmosphere 'too thin' for rivers

There may well have been water
on Mars. But a new study is arguing the red planet's thin atmosphere would
rarely have enabled it to remain in liquid form.

Before Mars Curiosity landed on the planet, satellite imagery
had already pointed to ancient water sources having been present.
And in the last six months alone, an ancient freshwater lake that could have hosted microbial life
was identified and an investigation into a 13kg meteorite found in
Antarctica demonstrated signs
of water flow on Mars. Visible ancient lakes combined with
prominent canyons suggest an ancient water flow that carved the
landscape we see on Mars today.

Now, a team headed up by Edwin Kite from Caltech California's
Geological and Planetary Sciences department is disputing these
suggestions, based on its identification of 319 small craters
visible along the ancient riverbeds near Gale crater. The size of
meteors that manage to penetrate a planet's atmosphere and make
craters is indicative of that planet's atmosphere, write the
authors, "because thinner atmospheres permit smaller objects to
reach the surface at high velocities and form craters".

Meteorites enter planetary atmospheres at high speeds, but slow
down when met by heat. The heat and atmospheric pressures cause the
object to break down to produce many smaller objects. If a large
enough object remains, it will create a crater on impact.

In relation to what we see on the surface of Mars, it would
stand to reason that if small rocks can get through undisturbed,
the atmosphere is thin. If the atmosphere is thick, the original
meteorite has to be much larger in order to survive the
descent.

All the team needed to do was reverse engineer the size of the
craters, working out the velocity of the meteorite and thus the
atmospheric conditions.

Using high-resolution images captured by the Mars Reconnaissance
Orbiter, the team made its calculations based upon our knowledge of
the surface of Mars, and by comparing that with our knowledge of
impacts that have occurred here on Earth.

The team concluded that around ten percent of the 319 craters
studied were 50m in diameter or under. The meteorites were
surviving the flight, and not breaking up as they would do if the
planet had a thick, warming atmosphere needed for liquid water to
thrive.

Using simulation software, the team repeatedly reproduced the
effects of meteorite impacts under different atmospheric pressures.
They found that 0.9 bar suited the formation of the craters best,
which is 150 times greater than the pressure on Mars today. This
would be good news, if it weren't for the fact the Sun was weaker
3.6 billion years ago, around the time the craters were formed.

In an accompanying paper published in Nature Geoscience, Sanjoy Som of
the Blue Marble Space Institute of Science and Nasa's Ames Research
Centre writes: "given that the early Sun was less luminous 3.6
billion years ago than it is today, atmospheric pressures of 5 bars
and above have been proposed to keep the surface above the freezing
point of water. The atmospheric pressure estimates of Kite et al
fall short of these predictions."

"If the small-crater limit is representative of early Mars, it
is difficult to envisage continuous stability of surface liquid
water for the 104 to 105 years needed to allow water to cycle
between deep aquifers and the surface," concludes the
team.

The team does, however, offer an alternative theory for all the
evidence of flowing water sources on the planet: "transient warming
by eruptions, impacts, or infrequent orbital conditions could
unfreeze the surface and shallow subsurface, allowing runoff, but
would not last long enough to unfreeze ground at less than 1km
depth." Here, they are referring to the warming greenhouse gases
that would have been released during impacts and volcanic
eruptions, and the occasional warming delivered when the planet
tilted on its axis and turned closer to the Sun during the 120,000
years it takes for an axis revolution to be completed.

If the Nature Geoscience study is right, Mars blew hot and
cold, but the former was far too infrequent to allow a watery
planet to thrive in liquid form.